As a main turbine in a nuclear power plant deteriorates due to secular changes, a turbine rotor or nozzle diaphragm is replaced by a new one to improve operation efficiency and enable longer utilization. During the replacement, radioactive substances adhering to an outer surface of the replaced old part such as a turbine rotor and nozzle diaphragm are decontaminated by blasting or the like and it is necessary to check the effect of decontamination by a radioactive contamination monitoring device so that the replaced part can be transported out of a radiation controlled area as domestic waste. On the other hand, surface metals removed by decontamination, sand containing radioactive substances, and parts from which radioactive substances cannot be removed due to a device structure or the like are packed in a drum and temporarily stored on a premise as low level radioactive waste before being transported to a low level radioactive waste burying center where the low level radioactive waste is buried in a concrete pit.
Because the space between nozzle wings to be monitored is very narrow, it has been very difficult to monitor for residual radiation of nozzle wings and the like after decontamination by blasting or the like using a commercial radioactive contamination monitoring device, prolonging a removal work period. For example, the space α between nozzle wings of a turbine rotor is about 3.9 mm to 24 mm and very narrow. Also, through-holes and non-through-holes formed on a horizontal joint surface of a turbine nozzle diaphragm have very small inside diameters.
Thus, to enable measurement of residual radiation of an object to be monitored whose surrounding space is narrow, a method of disassembling and pulling out a turbine rotor or cutting a nozzle diaphragm into three parts of outer rings, inner rings, and nozzle wings and then monitoring for residual radiation is commonly used. Because a nozzle wing is cast into an outer wing and inner wing, there has been no choice, but to mechanically cut or fuse for disassembly.
In one plant of 1100 MWh class, there are 8 stages×1 nozzle diaphragms into which the above nozzle wings are incorporated for a high-pressure turbine and 9 stages×3 nozzle diaphragms for a low-pressure turbine so that the number of nozzle plates amounts to about 10,000. A challenge is to reduce processes, save resources such as time, labor, and power, and also reduce installation costs of facilities by enabling storage of such nozzle plates as they are without cutting.
Fusing at high temperature is more efficient when compared with mechanical cutting, but radioactive substances may be melted and cured in a fusion zone, making work to grind/cut the fusion zone unavoidable to remove residual radioactive substances completely. Thus, it becomes necessary to reduce offcut (low level radioactive waste) in which radioactive substances are melted and generated by cutting, to be careful with fire, and to take safety measures including installation of a ventilator and a filter to remove fumes during cutting. Further, it is also necessary to secure a wide area within a limited radiation controlled area to make measurements with a radioactive contamination monitoring device.
Patent Literature 1 discloses a radioactive contamination monitoring device that enables easy monitoring for radioactive contamination of an inner surface of a tube. That is, a radiation detection unit configured by attaching scintillators or the like to a rod-like transparent guide line portion extended from a cylindrical photoelectric conversion unit and attaching a light shielding portion that allows radiation to pass through, but blocks light to an outer side thereof is disclosed. However, the radiation detection unit shown by Patent Literature 1 is intended for radiation measurement of an inner surface of a tube and has difficulty in monitoring a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow.